A turbomachine with integral gearing has a housing holding a shaft carrying both an impeller and a transmission input gear. Typically there are two such impellers axially flanking the gear so that axial forces are somewhat balanced. Axial-thrust bearings support the shaft and impellers in the housing. Such a device can be used as a turbocompressor or expansion turbine.
The turbocompressor rotor is driven by an integral gear transmission, while an expansion turbine rotor drives via an integral gear transmission either a load formed by a generator that is coupled to a large-diameter transmission gear or drives further turbocompressor rotors mounted around the common gear. The meshing teeth of the gears run obliquely to their rotation axis, generate when transmitting torque an axial force that must be countered in order to ensure meshing of the gears. At the same time, flow over the impellers generates axial forces that must be countered. As a rule, the axial forces are transmitted through the shafts on which the gears and impellers are attached to axial bearings and thence to the housing. The bearings that brace the rotor axially of the drive gear have a certain clearance.
If an axial force that is too strong acts on the axial bearings, the clearance is exceeded and axial bracing fails. This can lead to serious damage to the turbomachine, that is when the impeller blades brush against the closely adjacent mating surfaces of the housing. Contact between the impeller and housing thereby occurs either directly or indirectly via the seals of the impeller. In particular with turbomachines that either compress or expand pure oxygen, an impeller rubbing against the housing can have fatal consequences. The frictional generation of heat or sparking can trigger a catastrophic machine fire.
In order to avoid damage of this type, it is usual to detect axial displacement during operation of the rotor. When a limit is exceeded, suitable safety measures are taken. These can be either triggering an operating alarm or switching off the drive of the rotor. Both measures can also be used simultaneously.
Once the drive has been switched off, the turbomachine spins down. This spin-down can take a rather long time before the rotor actually stops. This is due to the generally very high operating speeds of turbomachines and their high mass moment of inertia. The entire drive train contributes to the high mass moment of inertia.
Due to the rotor mass once accelerated in the axial direction, axial forces also continue to act during the spin-down time. This leads to further axial displacement during spin-down so that damage to the turbomachine can still occur even after an emergency shut-down.